Hi Jennifer,
Sorry for the delay in reply, here it is:
I must admit that 8 days is not a lot to do both Hokkaido and the the major cities in Kansai and Hiroshima. Going up to Sapporo in Hokkaido takes about 7 hours by train from Tokyo. You'd basically need 2 travel days alone to make the trip up, then you will want at least 2-3 days to enjoy Hokkaido and you can see that there's not much time left for the rest. I would either do one of two options. 1.) Focus your itinerary on Tohoku and Hokkaido, these are the Northern parts of Japan. An 8 day itinerary could be, Tokyo - Sendai - Sapporo - Furano - Otaru - Hakodate - Tokyo or 2.) focus on visiting Kansai & Hiroshima. If you go trough most posts here on the forum, you'll see that people who visit Japan usually do a mix of Tokyo, Kyoto and Osaka within 7 days. Add Hakone and Hiroshima to that and you'll already be pretty well packed. So your itinerary could look like Tokyo - Osaka - Hiroshima - Kyoto - Hakone - Tokyo. You could get a great view from Hakone here.
You can always come back to Japan later if you want to see more :)
Hope this helps,
Daniel